Positron-annihilation studies of Frank-Kasper-type quasicrystals of the Mg-Zn-Ga-Al and the Mg-Zn-rare-earth system

We have performed positron-annihilation experiments thr a Frank-Kasper (F-K )-type icosahedral quasicrystal, 2/1 and 1/1 approximant crystals of Mg-Zn- Ga-Al, and the stable F-K-type icosahedral quasicrystals of Mg-Zn-RE (RE=ra re-earth metal, Y, Ho and Gd) alloys. It is shown that a high density of va cant centers in triacontahedral clusters exists in both icosahedral and app roximant phases of Mg-Zn-Ga-Al. Furthermore, in the present study, we repor t a very interesting finding that stable non-Al-based quasicrystals such as the icosahedral quasicrystal Mg-Zn-RE system also contain a high density o f structural vacancies, similar to the stable Al-based quasicrystals. In pa rticular, it is estimated experimentally that the structural vacancy densit ies for icosahedral quasicrystal Mg34Zn58Y8 and Mg40Zn52Ho8 are 3 x 10(20) and 4 x 10(20) cm(-3), respectively. These results provide important inform ation on the structural modeling of the stable icosahedral quasicrystal Mg- Zn-RE, which is 3 recent research topic of the icosahedral quasicrystals.